1. Field of the Invention
This invention is concerned with monitoring the security of a fiber optic communication system and with providing an alarm signal in the event of unauthorized eavesdropping thereon.
2. Discussion of the Prior Art
The advantages of optical fiber communication links are well known. Modern optical waveguides offer a low attenuation rate per kilometer, high data-transmission rates and a substantial bandwidth. The optical fibers are virtually free from electromagnetic interference and from radiation effects. Several channels of information may be transmitted bidirectionally over a single waveguide by wavelength-division multiplexing (WDM) techniques. A typical such system is taught by U.S. Pat. No. 4,628,501, issued 12/09/89 in the name of Claire E. Loscoe.
Although fiber optic systems are relatively more secure to surreptitious tapping than are wire lines, they can be compromised. Typically, in fiber optic communication links, an optical fiber and its cladding are coated with a compliant material that acts as a buffer against random environmental microbending and other mechanical distortions. Preferably, the coating may have a relatively hard outer shell and a soft inner layer. An evil-doer, intent on invading the communication system, would be obliged to penetrate a portion of the the buffer coating and to flex the fiber in such a manner as to couple the signals out of the fiber and into an evanescent coupler. That act will cause a phase shift of a data-modulated optical carrier signal propagating through the optical fiber. One method for making such a tap is disclosed in U.S. Pat. No. 4,632,513, issued in the name of D. W. Stowe et al.
It is the purpose of this invention to provide means for continuously monitoring the phase angle of a datamodulated optical carrier signal as measured relative to a reference signal and to sound an alarm if the measured relative phase angle departs significantly from a specified minimum.